Scientists are a step closer to understanding the causes of eczema and asthma after discovering a genetic mutation in the skin of people who have the conditions.

Scottish researchers have found that around two-thirds of eczema cases and a quarter of asthma cases involve mutations to a gene that helps form the skin's outer protective layer.

"[This] allows us for the first time to understand what goes wrong in the skin of these patients and paves the way for the development of new treatments," said Irwin McLean at the University of Dundee, who led the study. The group's research has brought hopes of cures for both conditions closer.

Eczema is a common skin condition which causes the skin to become inflamed and itchy. It affects around one in five children in Britain and is increasing.

Margaret Cox, the chief executive of the National Eczema Society, described the news as "a real step forward". She said it would allow clinicians to "tackle the cause rather than simply treat the symptoms of a previously 'incurable' skin condition".

Prof McLean's team studied four groups of people: 52 Irish children with eczema; 604 Scottish children with asthma; 372 Danish children whose mothers have asthma, and 15 families with a condition called ichthyosis vulgaris, which causes flaky and scaly skin. In all groups they found that two different mutations to a gene called fillagrin were much more common than in the general population, they report in Nature Genetics.

Although around one in 10 people of European origin carry one of the mutations, the study found that around two-thirds of the Irish eczema patients had it.

Fillagrin helps to form the skin's outer protective layer, which is made up of dead cells that are collapsed together into a continuous protein sheath that keeps water in and invaders such as bacteria out.

The fact that mutations to the fillagrin gene are over-represented in the disease group suggests that such changes make eczema more likely. This was backed up by the team's finding that virtually no individuals with two mutant copies of the fillagrin gene did not develop eczema.

"If you have two fillagrin mutations you are in an ultra-high risk group," said Prof McLean.

Both mutations switch the gene off, stopping it from producing the fillagrin protein. "They completely kill a gene," said Prof McLean. He added that people with one mutated copy make half as much fillagrin as normal and those with two mutated copies make none, explaining why they are more at risk.

But Prof McLean said that other environmental factors such as central heating or increased cleanliness might also play a role as some people with one mutated copy do not develop eczema at all

Around half eczema patients go on to develop asthma, accounting for about a quarter of asthma cases. Prof McLean is hopeful that a treatment that targets the fillagrin mutation might also be used to treat some forms of asthma.

About 5.2 million people in Britain receive treatment for asthma. The number of sufferers has increased by 50% in the past 30 years.